The present invention relates to a new and improved piezoid power supply that can supply power to the electronics located in a fast moving projectile which contains a programmable projectile fuze.
Piezo devices have been used for years as power supplies for various applications, including fuzes. When used in fuzes, the detonation energy required has been generated on set back and then used at the desired detonation time, usually from an impact switch. The energy generated by the piezoid in this way is dependent on only two parameters: (1) the specific material used; and (2) the compressive setback pressure applied. It does not depend on how many layers of bulk piezo material are used or on how these layers are interconnected.
Typically, the piezo energy source is made from one layer of a bulk piezo material, has a capacitance, C.sub.P, in the tens of pF's, and a relatively high voltage, V.sub.g, in the low hundreds of volts. The generated energy, E.sub.g, is thus given by the equation: EQU E.sub.g =1/2*C.sub.P *V.sub.g .sup.2
Next, the energy to be used for detonation is usually transferred to a storage capacitor, C.sub.s. The stored capacitor energy, E.sub.s, is given by the equation: EQU E.sub.s =1/2*C.sub.S *V.sub.S.sup.2
A substantial loss of generated energy occurs as a result of this transfer, but this loss is kept to a minimum if the storage capacitance and the capacitance of the piezoid are equal. In that case, 25% of the energy ends up in each capacitor and 50% is lost. This is illustrated in FIG. 2 where loss is plotted versus capacitance ratio.
The above arrangement works fine for detonator applications, since a detonator's response depends on the input energy and is unaffected by variations in capacitor voltage, i.e., whether a low or a high supply voltage is used. Therefore, it works well with the high voltage and low capacitance of a typical bulk piezoid.
On the other hand, the same is not true for the electronics used in a fuze. It is necessary to operate fuze electronics at low voltages. A typical range is 2.7 volts to 5.0 volts. This voltage range is much lower than voltages normally generated by piezoid power supplies.
A DC/DC converter cannot be used to reduce the piezoid power supply voltages to an acceptable level because none are available that can operate at voltages above 12 volts. Furthermore, a capacitor cannot be used to reduce the piezoid voltage to an acceptable level because too much energy loss occurs as a result of this transfer. By using a storage capacitor with a capacitance much greater than the piezoid capacitance, the piezoid voltage can be reduced to an acceptable level. However, although the voltage level is reduced to an acceptable level, the energy loss is so great (see FIG. 2), that only enough energy is left to power the electronics for a few hundred milliseconds.
Therefore, no previously known prior art devices involving piezoid power supplies can be used to power the fuze electronics located on a fast moving projectile.